Xcode 9 came with a re-engineered Source Editor, built entirely in Swift.
When you re-engineer something, it basically means starting from scratch and producing a similar product to before that has benefits from going through that process.
Doing this for the Source Editor resulted in faster scrolling, change font size of text via keyboard commands and a few other benefits.
The one omission from this whole re-engineering process is that they have not re-added collapsible methods.
What I mean by collapsible methods, is ability to shrink a method which could have hundreds of lines of code, down to a single line to take up less space on that file while editing.
If you had 10 methods and only need to work on one at a time for a while, if you collapse the other 9 methods, you only see roughly 10% of the code on that page, resulting in smoother performance, faster scrolling and most importantly, faster navigating through files.
This is a feature that is much more useful for developers that use massive methods such as myself. For example, the Kelly Pool gameplay screen uses a file that has 32 methods and almost 14,500 lines of code, and as soon as you get into the multiple thousands of lines of code, then performance starts to slow down slightly, making collapsable methods providing a significant difference to performance and navigation.